1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a method of manufacturing a cholesteric liquid crystal (CLC) color filter layer and a transmissive liquid crystal display device incorporating the CLC color filter layer manufactured according to the method.
2. Discussion of the Related Art
A liquid crystal display (LCD) device has been in the spotlight as a valuable, next generation display device because of its low power consumption and good portability.
An active matrix liquid crystal display (AMLCD) device, which includes thin film transistors as switching devices for each of a plurality of pixels, has been widely used because of its high resolution and fast moving images.
Because the LCD device is not luminescent, it needs an additional light source in order to display images. In general, the LCD device has a backlight behind a liquid crystal panel as a light source, and such an LCD is usually referred to as a transmissive LCD device. In the transmissive type, light incident from the backlight penetrates the liquid crystal panel, and the amount of the transmitted light is controlled according to the alignment of liquid crystal molecules. Because the transmissive LCD device uses the backlight as a light source, it can display a bright image in dark surroundings. However, the amount of the transmitted light is very small for the amount of light incident from the backlight. That is, because only about 7% of the light incident from the backlight is transmitted through the liquid crystal panel, the brightness of the backlight should be increased in order to increase the brightness of the LCD device. Consequently, the transmissive LCD device has high power consumption due to the backlight.
To solve the problems in the transmissive LCD device, a reflective LCD device has been proposed. In the reflective LCD device, sunlight or artificial light is used as a light source of the LCD device. The light incident from the outside is reflected at a reflective plate of the LCD device according to the arrangement of the liquid crystal molecules. Since there is no backlight, the reflective LCD device has much lower power consumption than the transmissive LCD device. By the way, the reflective LCD device generally includes an absorptive color filter layer, which is made of pigments or dyes, the same as the transmissive LCD device. The reflective LCD device also has a disadvantage of low light transmittance due to the absorptive color filter layer.
To improve the light transmittance in the reflective LCD device, a cholesteric liquid crystal (CLC) color filter has been researched and developed. As the CLC color filter selectively reflects and transmits light, the CLC color filter can emit light of high purity in color. Additionally, the CLC color filter functions both as a color filter layer and as a reflector. Therefore, since the reflective LCD device including the CLC color filter does not require an additional reflector, the number and duration of manufacturing processes are decreased and image quality is improved.
Liquid crystal molecules of the CLC are arranged in a helical structure. The helical structure is characterized by a helical direction and a pitch, which is a cycle of the helical structure. A color tone of light reflected by the CLC depends on the pitch.
The CLC color filter may be used in a transmissive LCD device. A transmissive LCD device including a CLC color filter will be explained in detail with reference to the following figures.
FIG. 1 is a plan view of a transmissive LCD device including a cholesteric liquid crystal (CLC) color filter according to the related art. As illustrated in the figure, first and second substrates 10 and 30 are spaced apart from and face each other. A cholesteric liquid crystal (CLC) color filter layer 12, which includes a first cholesteric liquid crystal (CLC) layer 12a and a second CLC layer 12b sequentially deposited, is formed on an inner surface of the first substrate 10. A common electrode 13 is formed on the CLC color filter layer 12.
The first and second CLC layers 12a and 12b are divided into sub pixels PR, PG and PB, each of which is a minimum unit for displaying an image. The first and second CLC layers 12a and 12b of each sub pixel PR, PG and PB have different helical pitches, and reflect light of colors corresponding to the helical pitches, respectively, wherein the helical pitches do not correspond to a displayed color at the sub pixel PR, PG and PB. For example, at a red (R) sub pixel PR, the first CLC layer 12a may have the helical pitch corresponding to green (G) and the second CLC layer 12b may have the helical pitch corresponding to blue (B). At a green sub pixel PG, the first CLC layer 12a may have the pitch corresponding to red and the second CLC layer 12b may have the pitch corresponding to blue. At a blue sub pixel PB, the first CLC layer 12a may have the pitch corresponding to red and the second CLC layer 12b may have the pitch corresponding to green. That is, the first CLC layer 12a may be controlled to have G, R and R pitches in order at the sub pixels PR, PG and PB and the second CLC layer 12b may be controlled to have B, B and G pitches sequentially at the sub pixels PR, PG and PB.
A first polarizer 14 is disposed on an outer surface of the first substrate 10. The first polarizer 14 acts as a CLC polarizer and selectively reflects one of left-handed circularly polarized light and right-handed circularly polarized light.
An array element layer 32 is formed on an inner surface of the second substrate 30. Although not shown in the figure, the array element layer 32 includes a thin film transistor as a switching element and a pixel electrode connected to the thin film transistor at each sub pixel PR, PG and PB. A hologram diffusion plate 34 is formed on the array element layer 32.
A liquid crystal layer 50 is interposed between the common electrode 13 and the hologram diffusion plate 34.
A retardation plate 36 and a second polarizer 38 are sequentially arranged on an outer surface of the second substrate 30. The retardation plate 36 changes circularly polarized light into linearly polarized light or changes linearly polarized light into circularly polarized light. The retardation plate 36 may be a quarter wave plate (QWP). The second polarizer 38 is a linear polarizer, and transmits linearly polarized light corresponding to a light transmission axis thereof.
When a Bragg's reflection condition is satisfied at the CLC color filter layer 12, desired light properties may be obtained. Ideal selective reflection/transmission effects may be expected for light less than 10 degrees with respect to a normal line to the CLC color filter layer 12.
The Bragg's reflection condition may be explained more in detail. If X-ray is incident on a lattice plane of a crystal, interference patterns are formed by the interference of the X-ray reflected at the lattice plane. If a distance between adjacent lattices is d, and an incident angle of the X-ray is 90-θ, the path difference for the X-rays reflected at a first plane and a second plane is 2d sin θ. If the path difference is an integer number of a wavelength of the X-ray, the reflected waves are strengthened by the reinforcement interference.2d sin θ=mλ(m=1, 2, 3, . . . )
If light having an incidence angle quarter than 10 degrees is incident on the CLC color filter layer 12, the reflected light has a wavelength different from a designed wavelength of the CLC color filter layer 12 according to the Bragg's condition. Thus, color purity of transmitted light is lowered.
Accordingly, in the related art transmissive LCD device, a high light-concentrating backlight, which may be referred to as a high collimating backlight, that concentrates light within about 10 degrees with respect to a line normal to a standard plane is widely used as a light source.
The high light-concentrating backlight unit 16 is disposed on the first polarizer 14, and more particularly, on a rear side of the first polarizer 14. The high light-concentrating backlight unit 16 includes a light-concentrating backlight 16b and a light-concentrating sheet 16a over the light-concentrating backlight 16b. Therefore, the high light-concentrating backlight unit 16 concentrates light in desired angles. That is, incident light is concentrated by the high light-concentrating backlight unit 16 to have an incident angle within less than 10 degrees, and the concentrated light transmits the CLC color filter layer, whereby the light has a corresponding color. Then, the light emitted through the hologram diffusion plate 34 should have viewing angles of about 140 degrees.
However, although the high light-concentrating backlight unit may be used, it is difficult to have the incident angle with the value less than about 10 degrees. Accordingly, bright color properties may be hard to obtain in the transmissive LCD device including the high light-concentrating backlight unit.